A specialized adaptation of electrochemical machining, known as shaped-tube electrochemical machining (STEM), is used for drilling small, deep holes in electrically conductive materials. STEM is a noncontact electrochemical drilling process which can produce holes with aspect ratios as high as 300:1. It is the only known method which is capable of manufacturing the small, deep holes used for cooling blades of efficient gas turbines.
The efficiency of a gas turbine engine is directly proportional to the temperature of turbine gases channeled from the combustor of the engine and flowing over the turbine blades. For example, for gas turbine engines having relatively large blades, turbine gas temperatures approaching 1500.degree. C. (2,700.degree. F.) are typical. To withstand such high temperatures, these large blades are manufactured from advanced materials and typically include state-of-the-art type cooling features.
A turbine blade is typically cooled using a coolant such as compressor discharge air. The blade typically includes a cooling hole through which the air passes. A further design advancement has been the addition of internal ridges in the cooling hole to effect turbulent flow through the hole and increase cooling efficiency. Cooling features within the hole such as turbulence promoting ribs, or turbulators, thus increase the efficiency of the turbine.
The cooling holes commonly have an aspect ratio, or depth to diameter ratio, as large as 300:1, with a diameter as small as a few millimeters. The turbulators extend from sidewalls of the hole into the air passage about 0.2 millimeters (mm), for example.
The method currently used for drilling the cooling holes in turbine blades is a shaped-tube electrochemical machining (STEM) process. In this process, an electrically conductive workpiece is situated in a fixed position relative to a movable manifold. The manifold supports a plurality of drilling tubes, each of which are utilized to form an aperture in the workpiece. The drilling tubes function as cathodes in the electrochemical machining process, while the workpiece acts as the anode. As the workpiece is flooded with an electrolyte solution from the drilling tubes, material is deplated from the workpiece in the vicinity of the leading edge of the drilling tubes to form holes.
Turbulated ridges are formed in the cooling holes by a modification of the standard shaped-tube electrochemical machining (STEM) process for drilling straight-walled holes. One common method is termed cyclic dwelling. With this technique, the drilling tube is first fed forward, and then the advance is slowed or stopped in a cyclic manner. The dwelling of the tool which occurs when the feed rate is decreased or stopped creates a local enlargement of the hole diameter, or a bulb. The cyclic dwelling causes ridges to be formed between axially spaced bulbs. Cyclical voltage changes may be required. These ridges are the turbulators.
The cyclic dwelling method is very low in process efficiency compared to shaped-tube electrochemical machining (STEM) drilling of straight-walled holes because of the long time required for drilling each bulb individually by cyclic tool dwelling. The dwell time required to form a single bulb can be greater than the time for drilling an entire straight-walled hole.
U.S. Pat. No. 5,306,401 describes a method for drilling cooling holes in workpieces comprising turbulator blades which uses a complex tool resetting cycle for each turbulator in the hole. It, too, has low process efficiency, having even longer operating times for drilling the turbulator ridges than the cyclic dwelling method because of the time required to reset the electrode tool.
In addition, both the cyclic dwelling method and the method disclosed in U.S. Pat. No. 5,306,401 require that additional equipment be used with a standard STEM machine for control of machine ram accuracy, and electrolyte flow and power supply consistency, since these are crucial to hole quality. Failure to control the dimensions of the turbulated holes often leads to part rejection, adding significant manufacturing costs to the machining process.
An improved electrode tool has been developed which provides for convenient, cost effective machining of features in holes with large aspect ratios. Examples of the features which may be produced on workpieces using the tool are turbulators in cooling holes in turbine airfoils, rifling in gun barrels, and grooves in air bearings. The tool is disclosed in aforementioned B. Wei et al., "A Method and Tool for Electrochemical Machining," U.S. application Ser. No. 09/187,663.
With the improved electrode, it is possible to simultaneously machine as many bulbs as desired, in whatever configuration desired, while achieving a significant reduction in process time. Furthermore, no variation of process parameters such as feed rate or voltage is needed, and therefore, costly computer controls for the instrument are not required.
The improved electrode of the above referenced patent application is composed of a coated surface in a pattern defining the features to be machined in a predrilled hole in a workpiece. Using a lathe to grind a pattern in the coated surface is difficult due to the small diameter of the improved electrode (as small as 1 mm) and thin walls, where the tube is hollow (as thin as 0.25 mm) In addition, residues of dielectric material left in the ground areas can affect the quality of the features produced using the electrode.
Accordingly, there is a need for a new and improved method for fabricating an electrode for use in a shaped-tube electrochemical machining (STEM) process. In particular, there is a need for a method for fabricating an electrode having a surface coated in a complex pattern.